1. Field of the Invention
This invention provides, generally, an apparatus and method for arbitrarily shifting the frequency of an incoming electromagnetic signal. More specifically, this invention pertains to the use of an optical source to up-convert, filter, and downconvert an input electromagnetic signal in the microwave spectrum to shift the frequency of the electromagnetic signal that is output.
2. Description of the Related Art
Currently microwave frequency shifters rely primarily on digital phase shifters for creating a serrodyne modulation format which results in a one-directional frequency shift, or the use of image rejection mixers which divide, modulate and recombine the signal, with the undesirable components (sidebands and carrier) removed by coherent cancellation. In the case of the digital phase modulator, the achievable image and carrier rejection is limited by the number of discrete bits that can be implemented in hardware, usually less than ten. This limits the achievable rejection to below 25 dB. In the case of the image rejection mixer, the achievable image and carrier rejection is limited by the ability to create two broadband microwave signals exactly 90.degree. out of phase with exactly the same amplitude (independent of the incoming frequency). Typical devices with 3.degree. phase error and a 0.25 dB amplitude imbalance are limited in their image and carrier rejection to 30 dB.
A summary of image rejection versus phase and amplitude imbalance is shown in FIG. 1. Note that the phase error of less than 1+ with no amplitude error is necessary for greater than 40 dB rejection. Further details of Serrodyne modulation is to be found in Klein et al.; The DIGILATOR, a New Broadband Microwave Frequency Translator; IEEE Transactions on Microwave Theory and Technology; Vol MTT-15, No. 3; pp. 172-179; March 1967; Cummings, The Serrodyne Frequency Translator; Proceedings of the IRE; Vol. 45, No. 2; pp. 195-186; February 1957; Thylen et al.; Electro-Optical Serrodyne Frequency Translator for .lambda.=1.3 .mu.m; IEEE Proceedings, Vol. 132 Pt.J, No. 2; pp. 119-121; April 1985; Johnson et al.; Serrodyne Optical Frequency Translation with High Sideband Suppression; Journal of Lightwave Technology; Vol. 6, No. 1; pp. 109-112; January 1988; and further details of image rejection mixers can be found in the commercial literature of Merrimac Industries, West Caldwell, N.J. for the Model Numbers SSF-2; SSP-1R, SSF-1E, SSP-2, SSM Series and SSB Series Single sideband modulators.
In general, electrical frequency shifters can operate only over a very narrow bandwidth due to engineering complexities. In the case of the single-sideband mixer, it becomes very difficult to phase and amplitude match over a broad bandwidth, therefore the presently available devices are usually limited to instantaneous bandwidths of less than 1-2 octaves at microwave frequencies. In the digital phase shifter, the trade off between creating many bits for good rejection is not compatible with broadband operation because of the many paths the signal takes, therefore the current devices are usually limited to instantaneous bandwidths of less than 12 GHz.
The fundamental ideas used in this invention of up-converting, filtering and downconverting to shift the microwave frequency has been achieved by the use of all-microwave components, as shown in FIG. 2. An incoming microwave signal, f.sub.sig, 12 is mixed with a microwave oscillator 14 having a frequency, f.sub.lock, in an up-converting double balanced mixer. The positive frequency component (upper sideband, USB) of the incoming signal 12 is frequency converted to f.sub.lock +f.sub.sig and the negative frequency component (lower sideband,LSB) of the incoming signal 12 is frequency converted to f.sub.lock -f.sub.sig. An electrical filter 18 attenuates either the shifted LSB or the shifted USB, as well as the incoming signal 12 and the mixing frequency, f.sub.lock. After mixing in the mixer 16, only the wanted (shifted) single sideband at the IF input, I, is applied to a downconverting mixer 22. A signal generated by a second oscillator 24, f.sub.a, is mixed with the shifted sideband in the mixer 22 to produce a signal with a new frequency f.sub.a -(.sub.lock -f.sub.sig), where the LSB is used without loss of generality. If F.sub.a -f.sub.lock is nonzero (if the oscillator at the frequency f.sub.a has a slightly different frequency than f.sub.lock), the resulting signal at f.sub.sig is shifted by f.sub.a -f.sub.lock. An example is where a 10.5 GHz input signal frequency is mixed with a 12 GHz oscillator to create frequencies of 1.5 and 22.5 GHz. A 5 GHZ low pass filter removes the original 10.5 GHz signal, the 12 GHz local oscillator and the 22.5 GHz shifted USB, leaving only the shifted LSB. The shifted LSB at 1.5 GHz is then upconverted in a second mixed with a 12 GHZ+500 Hz oscillator to a net shifted output frequency of 10.5 GHz+500 Hz. The results of this is shown in FIG. 3. If a traditional serrodyne or image rejection mixer were used there would be unwanted signals (images) at 10.5 GHz.+-.(n+1)*500 Hz, where n is a nonzero integer. The 63 dB carrier rejection and approximately 60 dB of image and spurious rejection shown in FIG. 3 is possible since the level of image and carrier rejection is determined by the level of filtering used between the mixers, this may be very high (&gt;100 dB). The drawback of all the microwave technique presented above is that to achieve wideband operation, a mixer with wide bandwidths on all three ports is required. Additionally, the intermodulated distortion products of electrical mixers are quite high and can lead to significant signal distortion.
This invention presents an apparatus and technique to obtain a frequency shifted microwave signal by utilizing electrically controlled optical phase shifters to create a serrodyne modulation or multiple offset-phase-locked lasers to shift the microwave signal to a new frequency. This results in an improved image and carrier rejection when compared to electrical techniques.